Honor design units in sketch dimensions and measurements (#241)

* Honor design units in sketch dimensions and measurements

* fix: apply document units to specialized length controls

* Accept explicit units in length fields; document design-unit dimension values

Length fields in a design-unit control now evaluate what is typed as a
length expression: a bare number stays in the design's units, while an
explicit unit ("10 mm", "0.5 in", "1 in + 5 mm") is converted.
Document that a bare numeric sketch.dimension value from scripts, the
control channel or MCP is read in the design's units.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

---------

Co-authored-by: bherbruck <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -37,6 +37,12 @@ rolls the numbering back with the design), so they can go straight into later co
 {"id":8,"method":"engine.execute","params":{"command":"sketch.dimension","params":{"entities":["l1"],"value":50}}}
 ```
 
+A bare numeric `value` given to `sketch.dimension` (from the control channel, scripts or MCP) is
+read in the design's units, exactly as if it were typed into the dimension box: in an inch design
+`"value": 2` makes the line 2 in (50.8 mm) long. Point coordinates stay in millimetres. To pin a
+unit regardless of the design, pass an expression with one: `"value": "50 mm"` or
+`"value": "0.5 in"`. Angles are degrees unless the expression says otherwise.
+
 ## UI
 
 | Method | Params | Notes |